1. Field of the Invention
The present invention relates to a spin-injection magnetic random access memory which performs magnetization reversal by use of spin-polarized electrons.
2. Description of the Related Art
In recent years, intensive attempts have been made to apply a magnetoresistive element utilizing a tunneling magneto resistance (TMR) effect to a magnetic random access memory (MRAM).
In recent years, a magnetoresistive element has been obtained whose magnetoresistive (MR) ratio indicating a magnetoresistive change ratio is 230% or more, and dependence of the MR ratio on a voltage has been suppressed. This increases a possibility of putting the magnetic random access memory to practical use.
When the magnetoresistive element is used as a memory element of the magnetic random access memory, one of two ferromagnetic layers sandwiching a tunnel barrier layer therebetween is used as a magnetic pinned layer whose magnetization direction is pinned, and the other layer is used as a magnetic free layer whose magnetization direction changes.
A memory element using such a ferromagnetic single or double tunnel junction has characteristics that it is possible to store data in a nonvolatile manner, a write/read time is as fast as 10 ns or less, and the number of write times is 1015 times or more.
However, in a case where an architecture is adopted in which a memory cell is constituted of one transistor and one magnetoresistive element, there is a problem that a cell size cannot be reduced to be not more than that of a dynamic random access memory (DRAM).
To solve this problem, there are proposed a diode type architecture in which the magnetoresistive element and a diode are connected in series between a bit line and a word line, a simple matrix type architecture in which the only magnetoresistive element is disposed between the bit line and the word line and the like.
However, in any case, an only magnetic field (current magnetic field) generated by a current pulse is used during writing (magnetization reversal) with respect to a magnetic recording layer. This causes new problems that power consumption at a write time increases, a memory capacity cannot be enlarged owing to a limit of a tolerable current density of a wiring line, and an area of a driver/sinker which generates the current pulse enlarges.
Therefore, a structure is proposed in which a thin film constituted of a high-permeability magnetic material (yoke material) is formed around a write line as a path of the current pulse.
According to this structure, since the high-permeability magnetic material applies a magnetic field to the magnetoresistive element with a good efficiency, a current value required for the writing can be lowered. However, the value of the write current cannot be reduced to 1 mA or less.
As a technology to solve these problems at once, there is proposed a write method by means of a spin-injection magnetization reversal process (see, e.g., U.S. Pat. No. 6,256,223).
In this spin-injection magnetization reversal process, spin-polarized electrons (spin-injection currents) are injected into the magnetic recording layer of the memory element to thereby reverse the magnetization of the magnetic recording layer. Since the current value of the spin-injection current required for the writing decreases with miniaturization of the magnetic recording layer, there is much hope for the process as the write method with respect to the magnetic random access memory.
However, in this method, a value of about 8×106 A/cm2 is required as the current density of the spin-injection current in order to perform the writing (magnetization reversal). Therefore, when the tunnel barrier layer is set to be thin with miniaturization of the magnetoresistive element, destruction of the layer comes into question (see, e.g., Yiming Huai et. al., Appl. Phys. Lett. 84 (2004) 3118 and 49th MMM conference ES-08, HA-05, and HA-12).
Therefore, the spin-injection currents need to be reduced in order to realize a stable write operation without causing such destruction.
Moreover, when the reduction of the spin-injection currents is realized in the spin-injection magnetization reversal process, the process can contribute to saving of the power consumption of the magnetic random access memory. In consequence, it is essential to reduce the spin-injection currents.